This was crashing using the clone tool without a clone image assigned.
Caused by {rB9111ea78acf4}.
Since above commit, `BKE_image_acquire_ibuf` was using `ima->runtime`
without checking for NULL first.
Since callers are not required to check for this, just return early
here.
note: there is still a memory leak using the clone tool without a clone
image assigned (but this was also the case before said commit and needs
to be investigated separately).
Maniphest Tasks: T93380
Differential Revision: https://developer.blender.org/D13377
236be8e9f1